Decoupling module for decoupling high-frequency signals from a voltage supply line

ABSTRACT

A decoupling module for decoupling high-frequency signals from a voltage supply line, the module including a plurality of parallel-connected capacitors (K 1 , K 2 , . . . ), which each have a capacitance (C 1 , C 2 , . . . ), and are characterized in that at least one of the capacitors (K 1 ) has an inductance (L 1 ) which is selected dependent on the capacitance (C 1 ) of the capacitor (K 1 ) and the voltage supply line inductance (L 12 ), so that a resonance is generated which compensates the self-resonance of the system from at least a further capacitor (K 2 , . . . ) and the entire voltage supply line (S). L 12  is the inductance of the voltage supply line running between the parallel-connected capacitors.

The invention relates to a decoupling module for decouplinghigh-frequency signals from a voltage supply line, the module comprisinga plurality of parallel-arranged capacitors which each have a certaincapacitance.

The design of high-frequency circuits as they are used in mobilecommunication nowadays is a highly complex and time-consuming process.In addition to the high-frequency pattern also the DC voltage supply ofthe active components and the digital signal processing of theintegrated circuit are to be optimized. In a module are found inaddition to DC voltage signals also low-frequency signals in the rangefrom 1 MHz to about 500 MHz, as well as high-frequency radio signalsfrom 1 GHz and beyond. Then there is the risk of undesired couplingsbeing created between the electric components and lines, which couplingscannot be taken into account during the design process and are notrecognized until the end of the development of a high-frequency module.

To be able to amplify high-frequency signals, the active components areto be connected to the central battery voltage from which they take thenecessary energy. A known problem at the end of the design process isthe crosstalk of high-frequency signals on the battery voltage supplylines. This coupling effect leads to feedback loops between the activecomponents. The behavior of the active components is then disturbedconsiderably and the whole high-frequency circuit may end in undesiredresonance.

Coupling the low-frequency and high-frequency signals into the voltagesupply lines cannot furthermore be avoided because all lines,miniaturized as they are, run very close together and are all connectedwith the same central battery voltage. More particularly the coupling oflow-frequency signals to the DC voltage lines is dangerous because thetransistors used in this frequency range have a high gain of about 20dB. A feedback loop with only −20 dB coupling thus leads to an overallamplification of 1. This is already sufficient for oscillation.

In order to avoid crosstalk of the signals from one active component toanother, the low and high-frequency signals on the voltage supply linesare grounded via a decoupling module. This decoupling module is to passall AC voltage signals to ground, but should not affect the DC voltage.In principle this is done with capacitors because no DC voltage can flowthrough them, so that the DC voltage of the battery remains unaffected.For the high-frequency signal the capacitor has an impedance Z thatdiminishes with the frequency

${Z = \frac{1}{j\;\omega\; C}},$where ω=2πf with f as the frequency of the high-frequency signal, C thecapacitance of the capacitor and j=√{square root over (−1)}. The higherthe frequency of the signal, the simpler that can be grounded via thecapacitor.

In many high-frequency circuits a large number of discrete ceramicmultilayer capacitors are used for decoupling the undesiredhigh-frequency signals from the DC voltage lines, which are solderedonto the high-frequency module. The one contact of the capacitors isconnected to the voltage supply line, the other to the ground line. Adisadvantage of these capacitors is the self-inductance L induced bytheir internal structure. The combination of the capacitor C and theinductance L leads to the fact that the effective decoupling capacitancediminishes with frequency and is zero at the frequency

$f_{C = 0} = \frac{1}{2\;\pi\;\sqrt{L \cdot C}}$With frequencies beyond f_(C=0) these capacitors function as a coil anddecoupling is then no longer guaranteed. When these capacitors are used,often in the design phase no satisfactory decoupling is reached andfurther time-consuming adaptations of the decoupling circuit arenecessary. These capacitors are predominantly used in mosthigh-frequency circuits to guarantee that no resonance arises.

To reduce the self-inductance, single-layer capacitors were developed.To reach a sufficiently high capacitance, either the layer thickness iskept very small (down to about 20 nm), or a material having a highdielectricity constant is chosen. Due to the much smallerself-inductance, the frequency f_(C=0) is considerably higher and thedecoupling of the high-frequency signal from the voltage supply line iseffective up to high frequencies. A disadvantage is that thesingle-layer capacitors are to be mounted as discrete components in manyapplications. Furthermore, the manufacturing and contacting of thinlayers is possible only in highly specialized and expensive processes.The materials used have a relatively high breakdown field strength ofabout 200 V/μm for typical thin-film ceramics up to 1000 V/μm forsilicon nitride. With very thin layers of about 20 nm for the siliconnitride this means that the breakdown field strength is reached at about20 V. In typical layer thicknesses in the range of 0.5 μm of thin-filmceramic capacitors the breakdown voltage is found at about 100 V. Thesecapacitors cannot thus be used in high-voltage ranges.

A further disadvantage of these thin-film capacitors is that they cannotbe manufactured in a thick-film process. The main part of the passive RFfunctions in RF front-end modules may be manufactured in LTCC technology(Low-Temperature Co-fired Ceramics) in three-dimensional multilayerceramic circuits in thick-film technology. It would be highlyadvantageous if also the decoupling module could be manufactured in thistechnology. In the thick-film technology, however, layer thicknesses of10 μm cannot be fallen short of. When capacitors with high capacitancesof various nanofarads are integrated, geometry-defined resonance of thecapacitor surfaces occurs. This resonance occurs at frequencies in thelower gigahertz range as a result of the higher layer thickness indielectrics in the thick-film technology. This resonance limits themaximum capacitance that can be integrated for decoupling at a givenfrequency. To nevertheless achieve a high overall capacitance, aplurality of capacitors can be connected in parallel to thus achieve acapacitance that is equal to the sum of the individual capacitances ofthe parallel-arranged capacitors. In the design of such a circuit as isshown, for example, in FIG. 1( a), connection lines between thecapacitors are necessary which themselves have an inductance. Thetransmission pattern shown in FIG. 1( b) shows that undesired resonanceoccurs that is below 1 GHz particularly in this range.

Therefore, it is an object of the present invention to render adecoupling module available that shows practically no resonance in thelower high-frequency range, particularly up to about 1 GHz.

This object is achieved by a decoupling module as claimed in claim 1.Advantageous embodiments are subject of the dependent claims. Thesubject of claim 6 is a multilayer stack that includes a decouplingmodule according to the invention.

According to the invention there is provided in a decoupling module ofthe type defined in the opening paragraph that to at least one of thecapacitors an inductance is assigned which is selected in dependence onthe capacitance of the capacitor, so that a resonance is generated whichcompensates the self-resonance of the system from at least a furthercapacitor and the voltage supply line. Surprisingly one thus manages toconstruct good decoupling modules, for example, also with capacitorsthat have a self inductance. Whereas the use of thin-film capacitorsrequires an ever smaller self-inductance of the capacitors and thushighly complex processes, the solution according to the inventiondescribes a simple way that makes it possible to manufacture decouplingmodules, for example, also in thick-film technology. This will befurther explained hereinafter.

The invention makes use of the fact that as a result of a specialarrangement of the parallel circuit of the capacitors, the undesiredresonance is compensated by a second resonance.

According to a preferred embodiment this may already be achieved if forat least two of the capacitors the relationshipC ₁ /C ₂ =L ₁₂ /L ₁holds, where L₁₂ is the inductance of the voltage supply line runningbetween the capacitors.

Basically, capacitors whose self-inductance is practically zero and cantherefore be neglected can be used in the invention. The invention,however, offers special advantages when capacitors with aself-inductance are used because, as described above, good decouplingmodules can be constructed also with these capacitors. Normally,non-ideal grounding could be expected if such capacitors were used.Thanks to the invention also such negative effects can be compensated.

This may be achieved, for example, if for at least two of the capacitorsthe relationC ₁ /C ₂=(L ₁₂ −L ₂)/L ₁holds, where L₁ is the self-inductance of the capacitor having thecapacitance C₁, L₂ the self-inductance of the capacitor having thecapacitance C₂ and L₁₂ is the inductance of the voltage supply linerunning between the capacitors.

In an advantageous manner the capacitance of one of the capacitorshaving a certain self-inductance can be selected such that aself-resonance generates a zero transmission at a further frequency.This may be, for example, an operating frequency of an active componentor a frequency of a neighboring module. With such a circuit branch theself-resonance of a further circuit branch is suppressed and itscapacitance may then be selected to be very large. Without the measure avery low self-resonance in the area of 200 MHz could be expected.

The decoupling module of the invention may be integrated with amultilayer stack known per se in which, for example, at least one layeris a dielectric layer having a relative permitivity ε>300 on which thecapacitors are positioned. Suitable layer thicknesses are found at about100 μm and below, preferably below 40 μm. The thickness of the layeraffects the situation of the geometry-bound resonance only within theframework of the dispersion.

The stack structure may then have one or also various layers having ahigh relative permitivity. Especially for the capacitors which accordingto the invention are allowed to have a self-inductance, a structure witheven more layers and thus higher capacitor density per surface ispossible.

Furthermore, with the invention it is possible to utilize as integratedcapacitors in circuits SMD (Surface Mounted Device) capacitors having ahigher self-inductance, and thus to achieve at least partly acost-saving, because expensive capacitors with minimized self-inductancemay be done without.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows the equivalent circuit diagram of a decoupling moduleaccording to the state of the art with a representation of thetransmission pattern;

FIG. 2 shows the equivalent circuit diagram of a decoupling moduleaccording to a first embodiment according to the invention with arepresentation of the transmission pattern;

FIG. 3 shows an equivalent circuit diagram of a further embodiment ofthe decoupling module according to the invention with a representationof the transmission pattern;

FIG. 4 shows an equivalent circuit diagram of a third embodiment of adecoupling module of the present invention with a representation of thetransmission pattern;

FIG. 5 gives a diagrammatic representation of a decoupling moduleaccording to the invention in which the capacitors are formed inthick-film technology, as well as a representation of the transmissionpattern;

FIG. 6 gives a representation similar to FIG. 5 of a decoupling moduleaccording to the state of the art; and

FIG. 7 gives a representation of a multilayer stack with a layer thathas a high dielectric constant and

FIG. 8 gives a representation of a multilayer stack having two layersthat have a high dielectric constant.

Different from the simple parallel circuit of capacitors as shown inFIG. 1, according to the invention an inductance is assigned to at leastone of the capacitors. Normally, this is not provided in decouplingmodules because one would like to connect the high-frequency portion toground, which is obstructed by inductance.

FIG. 2 shows an equivalent circuit diagram of a first embodiment of adecoupling module according to the invention in section (a). A firstcapacitor K, has not only a capacitance C₁ but also an inductance L₁.The second capacitor K₂, which is connected in parallel to the firstcapacitor K₁, has a self-inductance that can be neglected. Between thecapacitors K₁ and K₂ is inserted an inductance L₁₂ of the voltage supplyline S. The inductances L₁ and L₁₂ are to be selected such thatC ₁ /C ₂ =L ₁₂ /L ₁holds. The transmission pattern is shown in partial Figure (b). Itexactly matches the pattern of a capacitor having capacitance C₁+C₂ andshows no resonance in the frequency range below 2 GHz.

FIG. 3 shows in partial diagram (a) the equivalent circuit diagram of adecoupling module according to an embodiment of the invention in whichalso an inductance L₂ is assigned to the capacitor K₂. Otherwise thestructure corresponds to that of FIG. 2. In the partial Figure (b) whichshows the transmission property, a resonance f_(res,2) at 3.5 GHz can beseen, which corresponds to the resonance of the capacitor K₂ having theinductance L₂. The resonance of the capacitor K1 is compensated by thecircuit. The component values are to satisfy the relationC ₁ /C ₂=(L ₁₂ −L ₂)/L ₁

It is desirable for the capacitance C₂ of the capacitor K₂ to be keptsmallest possible. In that case the self-resonance with theself-inductance L₂ is not reached until comparable high frequencies arereached. As can be seen from partial Figure (b) the self-resonance ofthe circuit branch C₁L₁ is suppressed. The capacitance C₁ may thereforebe selected very large. Exemplary values for the components are statedin the Table 1.

TABLE 1 C₁ 5 nF L₁ 0.1 nH C₂ 0.2 nF L₂ 0.1 nH L₁₂ 2.4 nH

The principle according to the invention may be transferred to three ormore capacitors. An example of a circuit for three capacitors is shownin FIG. 4 in which the decoupling module of FIG. 3 is complemented by afurther capacitor K₃ connected in parallel. Such circuits make itpossible to achieve a decoupling capacitance enlarged by the factor 2 to3, without having to take resonance in the lower high-frequency rangeinto the bargain. The values for the individual capacitances C₁, C₂ andC₃ or inductances L₁, L₂, L₁₂ and L₃ respectively, may be retained bycomputer-supported simulations. The transmission pattern is shown inpartial figure (b).

FIG. 5 shows an example of a realization of the concept of the presentinvention in thick-film technology. Two planar capacitors K₁ and K₂ areconnected to a line S, with an inductance being generated by theconnection line L₁ between the capacitor K₁ and the line S. Thetransmission pattern in the partial Figure (b) shows no resonance in thelow-frequency range below 2 GHz.

FIG. 6 shows the ratios in an arrangement without a connection line andthus without additional inductance. The transmission pattern shown inpartial Figure (b) is clearly degraded.

FIG. 7 shows a cut-away view of a multilayer stack which shows inaddition to further layers S₁, S₂ . . . an integrated decoupling layerS_(ε), arranged as a layer having a high dielectric constant which istypically in the range from ε=300 to 2000. The thickness of this layerS_(ε) having a high dielectric constant is relatively small and istypically in the range from 10 μm to 40 μm, although also layerthicknesses up to 100 μm would be possible. Below the layer having thehigh dielectric constant there is the ground electrode S_(g) of thecapacitor which can completely or partly cover the layer S_(ε). On thelayer S_(ε) the planar capacitor fields K are embodied according to theprinciple shown in FIG. 5( a). This multilayer stack can be manufacturedin a thick-film process with ceramic layers.

FIG. 8 shows a variant in which two layers having a high dielectricconstant are provided between which the capacitor faces K of thedecoupling module are arranged. Ground electrodes S_(g1), S_(g2) arefound both on the surface of the upper dielectric layer S_(η1) and onthe lower surface of the lower dielectric layer S_(ε2). With theinvention the enlarged capacitance can be utilized. The inventionrepresents an extensive possibility of manufacturing decouplingcapacitances for the lower high-frequency range in a standard thick-filmprocess. This provides that the complex thin-film process can be donewithout for the decoupling function. Furthermore, the limitation of thecapacitance by geometry-bound resonance of the capacitor surfaces can beovercome.

Also with relatively high self-inductances, decoupling capacitors can bemanufactured in which no resonance occurs in the lower high-frequencyrange. This property of the capacitors makes it possible for therequirements as to the grounding to be mitigated, because thecompensation for the non-ideal grounding can be introduced with thecircuit.

The higher film thickness in thick-film processes makes it possible forthe decoupling module according to the invention to be used for highervoltages. The breakdown field strength is reached only at voltages ofseveral hundred volts. The subdivision of the whole decouplingcapacitance into a plurality of individual capacitances is also anoption for thin-film decoupling capacitors for high capacitances. Inthis way the geometry-bound resonance can be compensated also withlarger dimensions.

Also for connecting together a plurality of thin-film capacitors thecircuitry proposed here can be maintained because low-frequencyresonance occurs as a result of the connection lines between thecapacitors. High self-resonance is not sufficient for a suppression ofsuch resonance.

1. A decoupling module for decoupling high-frequency signals from avoltage supply line, the module comprising a plurality ofparallel-connected capacitors (K₁, K₂, . . . ) which have eachcapacitance (C₁, C₂, . . . ); wherein an inductor (L₁₂, L₂₃, . . . ) isthe inductance of the voltage supply line running between oneparallel-connected capacitor, and a next parallel-connected capacitor;wherein at least one of the capacitors (K₁, K₂, . . . ) has a seriesinductance (L₁, L₂, . . . ) which is selected dependent on thecapacitance (C₁, C₂, . . . ) of the capacitor (K₁, K₂, . . . ), and thevoltage supply line inductance, so t hat a resonance is generated whichcompensates self-resonance of the system from at least a furthercapacitor (K₂, K₃, . . . ) and the entire voltage supply line (S).
 2. Adecoupling module as claimed in claim 1, characterized in that for atleast two of the capacitors (K₁, K₂) the relationship C₁/C₂=L₁₂/L₁holds, where L₁₂ is the inductance of the voltage supply line (S)running between the capacitors (K₁, K₂).
 3. A decoupling module asclaimed in claim 1, characterized in that for at least two of thecapacitors (K₁, K₂) the relationship C₁/C₂=(L₁₂−L₂)/L₁ holds, where C₁is the capacitance and L₁ the inductance of the capacitor (K₁) and C₂the capacitance and L₂ the inductance of the capacitor (K₂) and L₁₂ isthe inductance of the voltage supply line running between the capacitors(K₁, K₂).
 4. A decoupling module as claimed in claim 1, characterized inthat the capacitance (C₂) of one of the capacitors (K₂) having theself-inductance L₂ is chosen such that its self-resonance (f_(res,2))generates a transmission zero-crossing at a further frequency.
 5. Adecoupling module as claimed in claim 1, characterized in that thecapacitors are thick-film capacitors.
 6. A decoupling module as claimedin claim 1, characterized in that at least one of the capacitors is anSMD capacitor (Surface Mounted Device) having self-inductance.
 7. Amultilayer stack comprising a decoupling module as claimed in claim 1,in which at least one layer is a dielectric layer having a relativepermitivity ε≧300 on which the capacitors are mounted.
 8. A method ofproviding decoupling of high-frequency signals from a voltage supplyline comprising, coupling a plurality of parallel-connected capacitors(K₁, K₂, . . . ) which have each a capacitance (C₁, C₂ . . . ), whereinan inductor, (L₁₂, L₂₃ . . . ) is the inductance of the voltage supplyline running between one parallel-connected capacitor, and a nextparallel-connected capacitor, wherein at least one of the capacitors(K₁, K₂ . . . ) has a series inductance (L₁, L₂ . . . ) which isselected dependent on the capacitance (C₁, C₂ . . . ) of the capacitor(K₁, K₂ . . . ), and the voltage supply line inductance; and, generatinga resonance which compensates the self-resonance of the system from atleast a further capacitor (K₂, K₃ . . . ), and from the entire voltagesupply line (S).
 9. A method, according to claim 8, w herein for atleast two of the capacitors (K₁, K₂) the relationship C₁/C₂=L₁₂/L₁holds, where L₁₂ is the inductance of the voltage supply line (S)running between the capacitors (K₁, K₂).
 10. A method, according toclaim 8, wherein for at least two of the capacitors (K₁, K₂) therelationship C₁/C₂/=(L₁₂−L₂)/L₁ holds, where C₁ is the capacitance andL₁ the inductance of the capacitor (K₁) and C₂ the capacitance and L₂the inductance of the capacitor (K₂) and L₁₂ is the inductance of thevoltage supply line running between the capacitors (K₁, K₂).
 11. Amethod, according to claim 8, wherein the capacitance (C₂), of one ofthe capacitors (K₂) having the self-inductance L₂, is chosen such thatits self-resonance (f_(res,2)) generates a transmission zero-crossing ata further frequency.
 12. A method, according to claim 8, wherein thecapacitors are thick-film capacitors.
 13. A method, according to claim8, wherein at least one of the capacitors is an SMD capacitor (SurfaceMounted Device) having self-inductance.
 14. A method, according to claim8, further comprising, providing a multilayer stack in which at leastone layer is a dielectric layer having a relative permittivity ε≧300 onwhich the capacitors are mounted.